su: Fix never alarmed SIGKILL when session terminates

The buggy code was introduced nearly 5 years ago at the
commit 08fd4b69e8. The
desired behavior is that SIGKILL will be sent to the
child if it does not exit within 2 seconds after it
receives SIGTERM. However, SIGALRM is masked while
waiting for the child so it cannot wake the program
up after 2 seconds to send SIGKILL.

An example shows the buggy behavior, which exists in
Ubuntu 18.04 LTS (with login 1:4.5-1ubuntu2).
```bash
user1@localhost:~$ su user2 -c '
_term() {
  echo SIGTERM received
}
trap _term TERM

while true; do
  sleep 1
  echo still alive
done'
Password:
still alive

Session terminated, terminating shell...Terminated
SIGTERM received
still alive
still alive
still alive
still alive
```
(SIGTERM is sent in another user1's terminal by
executing `killall su`.)

Here is the desired behavior, which shows what the
commit fixes.
```bash
user1@localhost:~$ su user2 -c '
_term() {
  echo SIGTERM received
}
trap _term TERM

while true; do
  sleep 1
  echo still alive
done'
Password:
still alive

Session terminated, terminating shell...Terminated
SIGTERM received
still alive
still alive
 ...killed.
user1@localhost:~$ echo $?
255
```

src/su.c

@@ -397,22 +397,28 @@ static void prepare_pam_close_session (void)
 		snprintf (kill_msg, sizeof kill_msg, _(" ...killed.\n"));
 		snprintf (wait_msg, sizeof wait_msg, _(" ...waiting for child to terminate.\n"));
 
+		/* Any signals other than SIGCHLD and SIGALRM will no longer have any effect,
+		 * so it's time to block all of them. */
+		sigfillset (&ourset);
+		if (sigprocmask (SIG_BLOCK, &ourset, NULL) != 0) {
+			fprintf (stderr, _("%s: signal masking malfunction\n"), Prog);
+			kill_child (0);
+			/* Never reach (_exit called). */
+		}
+
+		/* Send SIGKILL to the child if it doesn't
+		 * exit within 2 seconds (after SIGTERM) */
 		(void) signal (SIGALRM, kill_child);
 		(void) signal (SIGCHLD, catch_signals);
 		(void) alarm (2);
 
-		sigemptyset (&ourset);
-		if ((sigaddset (&ourset, SIGALRM) != 0)
-		    || (sigprocmask (SIG_BLOCK, &ourset, NULL) != 0)) {
-			fprintf (stderr, _("%s: signal masking malfunction\n"), Prog);
-			kill_child (0);
-		} else {
-			while (0 == waitpid (pid_child, &status, WNOHANG)) {
-				sigsuspend (&ourset);
-			}
-			pid_child = 0;
-			(void) sigprocmask (SIG_UNBLOCK, &ourset, NULL);
+		(void) sigdelset (&ourset, SIGALRM);
+		(void) sigdelset (&ourset, SIGCHLD);
+
+		while (0 == waitpid (pid_child, &status, WNOHANG)) {
+			sigsuspend (&ourset);
 		}
+		pid_child = 0;
 
 		(void) fputs (_(" ...terminated.\n"), stderr);
 	}